A typical optical receiver (Rx) includes at least one photodiode that detects an optical signal and converts it into an electrical signal and at least one transimpedance amplifier (TIA) that converts the electrical signal into a voltage signal. The photodetector, which is typically a P-intrinsic-N (PIN) photodiode, produces an electrical current signal in response to light detected by the photodetector. The TIA converts this electrical current signal into an output voltage signal having some gain, commonly referred to as transimpedance gain. This output voltage signal is further processed by other stages (i.e., a limiting amplifier (LA), clock and data recover (CDR), etc.) of the Rx.
The TIA handles input signals (the photodiode output) of varying optical modulation amplitude (OMA) and average power (Pavg), and therefore needs to have a wide input dynamic range. Wide input dynamic range is typically achieved by incorporating an automatic gain control (AGC) circuit in the RX portion for automatically adjusting the gain of the TIA based on the amplitude of the input signal. If the Rx does not include an AGC circuit, the TIA of the Rx will try to use its transimpedance gain to convert the current into a corresponding output voltage as the amplitude of input current signal increases. When this happens, however, the transimpedance gain is limited by the voltage headroom (the maximum high and low output voltage for linear operation of the TIA) as the output voltage swing increases, which results in the output signal becoming distorted. Hence, an AGC circuit is needed in order to lower the gain of the TIA as the amplitude of the input signal grows so as to prevent the TIA from saturating and producing distortion at its output.
Burst-mode optical Rxs are used in networks in which optical signals of various optical power levels and phases (timeslots) are transmitted from various sources. The TIA used in a burst-mode optical Rx should be capable of handling such optical signals. Although it is known to use AGC circuits in burst-mode optical Rxs for automatically adjusting the gain of the TIA based on the incoming signal, existing solutions generally have large pulse-width distortions and limited dynamic range, especially for the first bit received after a long period of quiescence.
Moreover, existing solutions typically require transmission of a training bit sequence to the Rx prior to the data being transmitted. The training bit sequence is then processed in the Rx to set the TIA gain and the decision threshold value. Use of the training bit sequence increases processing overhead and reduces the effective data rate of the optical link.
Accordingly, a need exists for a burst-mode Rx that is capable of adapting both the gain and the decision threshold simultaneously and very quickly to obviate the need to transmit and receive a training bit sequence.